Assuming the detector coil is functioning properly, a target that 'disappears' can be a poor conductor that's altered its orientation with respect to the magnetic field when being dug and now seems to disappear. Some detectors are more prone, such as the CTX which has trouble with weak conductors. Another source is simply EMI. The same test inside a house compared with away from electronics can yield better sensitivity and depth where EMI is reduced or eliminated. With the CTX, iron overwhelms nearby non-ferrous due to its much stronger field strength relative to non-ferrous that the CTX will often miss the co-located non-ferrous or only hit the non-ferrous in one sweep direction. Digging out the target can separate the targets enough to reveal the non-ferrous clearly which makes it appear like the detector had lost depth in the ground. Too aggressive use of Disc can silence marginal targets (especially in mineralized soils) that also appear to sound off when dug. The target signal breaks through when it rights itself (down-averages toward lower FE when the dirt is removed or target is brought to the surface) once freed from being up-averaged (pulled to higher FE values by the iron minerals or simply iron nails present which the Disc silences with the target). But target orientation is very important, sometimes even on the surface, a low-conductor disappears because of a non-preferential angle to the coil (in the water, a light non-ferrous target can float away drawn by the seabed currents, so it actually does disappear on its own). And a small percentage of CTX targets have strong signals in the ground, disappear in the dirt pile, and never are recovered (not in the hole either). Those are the mystery ones that seem to have no explanation where the good target went to (pinpointer can't find any trace. Likely the target fell to pieces while digging, that is most probable, indicating it was old rusted iron that disintegrated when dug causing the signal to disappear -- look for evidence of rust in the dirt).
